This invention relates to a leading/trailing type drum brake assembly in which the parking brake lever, adjuster strut and associated components are replaced with a single electronic subassembly that controls the parking brake, senses brake lining wear and adjusts the brake shoe-to-drum clearance to account for lining wear. The parking brake assembly includes a strut actuator subassembly that comprises a strut tube, an electric motor, a position encoder, a power screw and a connector fork.
Vehicle brakes are used to slow the vehicle and also to maintain the vehicle in a parked position. Drum brakes are a common type of vehicle brake. Drum brake assemblies utilize opposing arcuate brake shoes that are movable away from one another into engagement with an inner surface of a drum to slow the vehicle. Typically, a hydraulic wheel cylinder forces the brake shoes into engagement with the drum. The same brake shoes may also be used to maintain the vehicle in a parked position. For example, drum brake assemblies have employed a separate mechanical linkage, or parking brake lever, that is actuated by a cable to maintain the brake shoes in engagement with the drum. Such mechanical parking brakes have presented various problems that have been partially addressed by the use of electric parking brake mechanisms.
Some electric parking brake mechanisms have eliminated the parking brake lever and completely redesigned the drum brake assembly to incorporate an electric parking brake mechanism. These mechanisms have required high torque motors and a gearbox to generate sufficient force to apply the brakes, adding significant cost to the drum brake assembly. These high torque motors tend to be relatively large and cannot simply replace components presently included in brake drum assemblies, but instead require substantial redesign of the brake assembly package. Furthermore, high torque motors utilize a significant amount of power that is undesirable in modern vehicles.
Presently available electric brake systems have the further disadvantage that they rely on conventional brake adjustment mechanisms. Such conventional adjustment systems are mechanical and only operate when the wheels are moving in a reverse direction. In addition, neither known electric brake systems nor conventional brake systems provide real-time sensing of brake lining wear and determination of a worn brake lining condition.
Therefore, an electric parking brake assembly is needed that requires relatively little power to apply, that may be easily retrofitted into a conventional drum brake assembly, and that provides real-time adjustment of the brake shoe-to-drum clearance and sensing of brake lining wear.
The present invention is directed to an electrically and hydraulically (hereinafter referred to as xe2x80x9celectro-hydraulicxe2x80x9d) actuated electronic parking brake assembly for a leading/trailing type automotive drum brake. The parking brake assembly includes a strut actuator subassembly that replaces conventional components including the parking brake lever, adjuster strut and associated parts, used in a standard drum brake. The strut actuator subassembly is easily incorporated into a conventional brake assembly and comprises a strut tube, an electric motor having a driveshaft, a position encoder, a power screw and a connector fork. Due to the design of the strut actuator subassembly, a low power (and low torque) motor can be used.
The brake assembly of the present invention includes a backing plate having primary and secondary shoe hold-down pin and spring assemblies with primary and secondary brake shoes. Each of the brake shoes has a portion in spaced relation from one another. A drum is disposed about the brake shoes. A wheel cylinder is supported by the backing plate and is arranged between the portions of the brake shoes for forcing the portions away from one another to engage the brake shoes with the drum in a brake position. A strut actuator subassembly, located adjacent to the wheel cylinder, interconnects the brake shoes proximate to the portions. The strut actuator subassembly comprises a strut tube, a power screw, a connector fork, an electric motor with a driveshaft and a position encoder. The strut tube has one open end having female threads along a portion of the interior and an opposing end that tapers to a two-pronged fork. The power screw has male threads along a portion of its length and an unthreaded rod portion at one end. The connector fork has an annular female receiver portion at one end suitable for receiving the rod portion of the power screw and a two-pronged fork at the other end. The electric motor and position encoder are located within the strut tube, and the electric motor is axially connected with the power screw via the driveshaft. The strut tube and the power screw connect at their respective female and male threaded ends. The power screw, at its rod end, connects to the female end of the connector fork. The forked end of the strut tube and the connector fork connect with slots in portions of each web of opposing brake shoes thereby securing the strut actuator subassembly between the opposing brake shoes. The strut tube, electric motor with driveshaft, position encoder, power screw and connector fork are in axial relationship with one another and when connected define the transverse structure of the strut actuator subassembly.
In operation of an electro-hydraulic apply parking brake of the present invention, the strut actuator subassembly serves as a parking brake latch. Hydraulic pressure is applied by the electro-hydraulic actuator to the wheel cylinder to advance the brake shoes against the drum. As the brake is applied hydraulically, the electric motor of the strut actuator subassembly is energized. The strut actuator subassembly turns the power screw to advance the connector fork and strut tube out against the hydraulically extended brake shoes. Due to the design of the power screw, it can only be driven in positive direction. Accordingly, when the hydraulic pressure to the wheel cylinder is released along with the power to the electric motor, the brake shoes become locked to against the drum. The parking brake is released by the application of a hydraulic pressure sufficient to remove the axial load on the power screw, the connector fork and the strut tube. The electric motor then re-energizes, reverses and retracts the power screw and strut tube away from the shoes allowing them to return to their predetermined disengaged position when the hydraulic pressure is removed. The connector fork returns along with the return of the brake shoes by action of the return springs.
The present invention also advantageously provides regular and precise brake adjustment and sensing of braking lining wear. During operation, the electric motor is periodically energized, such as during the ignition start cycle, causing the powers crew to turn which advances the brake shoes out until they touch the drum. The motor then reverses for a predetermined number of counts of the position encoder to obtain the desired shoe-to-drum clearance. This provides regular and precise brake adjustment. The position encoder also concurrently determines the distance that the shoe has traveled and compares it with the original brake lining position to provide a reasonable estimate of a worn out lining condition. Upon such indication of wear, the position encoder sends out a signal to notify the operator of the worn lining condition.
Accordingly, the above invention provides an electric parking brake mechanism that provides regular and precise brake adjustment and lining wear indication. The mechanism requires relatively little power to operate and may be easily incorporated, such as by retrofit, into a conventional drum brake assembly.